1. Technical Field
This invention relates to integrated circuit manufacturing processes and, more specifically, to a novel process using organohalide compounds for the etching of metal films to produce organometallic etch byproducts.
2. Background
In order to build an integrated circuit, many active devices need to be fabricated on a single substrate. The current practice in semiconductor manufacturing is to use thin film fabrication techniques. Etching, in semiconductor applications, is a fabrication process used to produce patterns by which material is removed from the silicon substrate or from thin films on the substrate surface. A mask is typically used to protect desired surface regions from the etchant and this mask is stripped after the etching has been performed. The composition and uniformity of these thin layers must be strictly controlled to facilitate etching of submicron features.
As the pattern dimensions approach the thickness of the films being patterned, it is increasingly required that a dry etching process be selected. A number of dry etching process types exist. The process that has many applications in the fabrication of integrated circuits and that is widely used is plasma dry etching.
The basic concept of plasma dry etching is rather direct. A plasma is defined to be a partially ionized gas composed of ions, electrons, and a variety of neutral species. A glow discharge is a plasma that exists over a pressure range of 1 mtorr to 10 torr, containing approximately equal concentrations of positive particles (positive ions) and negative particles (electrons and negative ions). In plasma etching applications the glow discharge can be used to produce energetic ionic bombardment of the etched surface. However, the glow discharge has another even more important role, that of producing reactive species, such as atoms, radicals and ions from a relatively inert molecular gas, for chemically etching the surfaces of interest. A radical is an atom, or collection of atoms, which is electrically neutral, but which also exists in a state of incomplete chemical bonding, making it very reactive. The radicals, in fact, are responsible for most of the actual chemical etching phenomena that occur at the surface of the material being etched.
The characteristics of different etch processes vary widely with the process parameters, especially the gas composition. The gases adopted for plasma etching processes have traditionally been selected on the basis of their ability to form reactive species in plasma, which then react with the surface materials being etched and lead to volatile products. Traditionally molecules like carbon tetrachloride (CCl.sub.4) and carbon tetrafluoride (CF.sub.4) have been used because in a plasma reaction they produce the desirable reactive species which, in various combinations with other gases, etch silicon, silicon oxides, and other materials used in the manufacture of integrated circuits. Whereas heavily hydrogenated, organic analogs of these molecules, like methyl chloride (CH.sub.3 Cl) or methyl fluoride (CH.sub.3 F), have been considered to be undesirable starting materials because they tend to form polymers which interfere with the desired etch reaction.
The fluorine-containing gases used to etch Si and SiO.sub.2, however, are not suitable for etching aluminum since the etch byproduct, AlF.sub.3, has an unacceptably low vapor pressure. The etching of aluminum and aluminum alloy films is a very important step in the fabrication of integrated circuits. Other inorganic halide etch byproducts of aluminum (Al), such as AlCl.sub.3, have sufficiently high vapor pressures to allow plasma etching of Al, and thus chlorine-containing gases have been exploited to develop dry-etch processes for aluminum films. There are drawbacks to using chlorine containing gases to etch aluminum, however, as they are all either carcinogenic or highly toxic and possess other disadvantages in the fabrication process, such as destroying the efficiency and life span of the pumps and other hardware used for plasma etching.
The deposition of the AlCl.sub.3 etch byproduct on the plasma chamber walls also has deleterious effects on the etch process, so minimizing its deposition is important. Techniques used to manage AlCl.sub.3 in plasma etching applications have traditionally included maintaining the temperature in the etch chamber above 35.degree. C. (as the AlCl.sub.3 evaporation rate at such temperatures is high enough to assist in removing it from the chamber), and using large gas flow rates to keep its partial pressure low. In general, the inorganic etch byproducts produced by the dry etching of aluminum and other metals are very stable with high heats of formation and relatively low vapor pressures, leading to low etch rates and lower production throughputs.
The economics of integrated circuit fabrication demand the highest throughput obtainable within a given set of production standards. On the whole, the demands of high throughput require the highest etch rate consistent with good results. What is still needed is a dry plasma etch process that allows for an improved, higher etch rate of metals for the semiconductor fabrication etch process.